1. Field of the Invention
The present invention relates to a semiconductor device and more particularly, to a semiconductor device containing diodes connected in parallel for protecting Integrated circuits (ICs) against electrostatic charges.
2. Description of the prior art
FIG. 1 shows an example of conventional semiconductor devices of this sort, in which a main P-N junction is formed by diffusion processes.
In FIG. 1, a voltage regulator diode or Zener diode D21 is provided on a first main surface of an N-type silicon substrate 20a to protect ICs (not shown) to which the diode D21 is connected against electrostatic charges.
A silicon dioxide film 23a having a window is formed on the first main surface of the substrate 20a. A P-type diffusion region 21a is formed in the substrate 20a along the periphery of the window to act as a guard ring for the diode D21. The circular-ringed diffusion region 21a is deposited in the surface. area of the substrate 20a adjacent to the first main surface.
In the surface area of the substrate 20a, there are formed a P.sup.+ -type diffusion region 22a for forming a p-n junction at a corresponding position to the window of the silicon dioxide film 23a. The circular diffusion region 22a is surrounded by the diffusion region 21a as the guard ring.
An anode 24a of the diode D21, which is made of a patterned conductive film, is formed on the exposed part of the substrate 20a to be in contact with its first main surface through the window of the silicon dioxide film 23a.
The N-type silicon substrate 20a and the P.sup.+ -type diffusion region 22a form the main P-N junction of the diode D21 at their contact area surrounded by the guard ring 21a. The fringe of the anode 24a is deposited on the silicon dioxide film 23a.
The P.sup.+ -type diffusion region 22a is larger in impurity or acceptor concentration than the P-type diffusion region 21a. The impurity concentration of the P.sup.+ -type diffusion region 22a is determined to obtain a desired breakdown voltage of the main P-N junction.
A cathode 25a of the diode D21, which is made of a patterned conductive film, is formed on the second main surface of the substrate 20a, which is opposite to the first main surface.
The anode 24a and the cathode 25a are electrically connected to given circuits (not shown) of the ICs formed on the substrate 20a, respectively, and the ICs thus electrically connected are packaged.
FIG. 2 shows another example of the conventional semiconductor devices of the sort, in which a main P-N junction is formed with a polycrystalline semiconductor film.
In FIG. 2, a Zener diode D22 is provided on a first main surface of an N-type silicon substrate 20b to protect ICs (not shown) to which the diode D22 is connected against electrostatic charges.
A silicon dioxide film 23b having a window is formed on the first main surface of the substrate 20b. A P-type diffusion region 21b is formed in the substrate 20b along the periphery of the window to act as a guard ring for the diode D22. The circular-ringed diffusion region 21b is deposited in the surface area of the substrate 20b adjacent to the first main surface.
A P.sup.+ -type patterned polysilicon film 22b is formed on the first main surface of the substrate 20b through the window of the silicon dioxide film 23b. The polysilicon film 22b is in contact with the exposed part of the first main surface through the window to form the main P-N junction of the diode D22. In other words, the main P-N junction is formed at the contact area between the P.sup.+ -type polysilicon film and the N-type silicon substrate 20b and is surrounded by the diffusion region 21b as the guard ring. The fringe of the polysilicon film 22b is deposited on the silicon dioxide film 23b.
An anode 24b of the diode D22, which is made of a patterned conductive film, is formed on the P.sup.+ -type polysilicon film 22b.
The P.sup.+ -type polysilicon film 22b is larger in impurity or acceptor concentration than the P-type diffusion region 21b. The impurity concentration of the P.sup.+ -type polysilicon film 22b is determined to obtain a desired breakdown voltage of the main P-N junction.
A cathode 25b of the diode D22, which is made of a patterned conductive film, is formed on the second main surface of the substrate 20b, which is opposite to the first main surface.
The anode 24b and the cathode 25b are electrically connected to given circuits (not shown) of the ICs formed on the substrate 20b, respectively, and the ICs thus electrically connected are packaged.
In general, Zener diodes for protecting ICs against electrostatic charges are required to have the following electric characteristics (1) to (4).
(1) The breakdown voltage is higher than input/output signal voltages of the ICs and is lower than the withstand voltage of the ICs. For example, in case the input/output signal voltages are 5 V, the breakdown voltage should be about 6 V.
(2) The leakage current is small for restricting distortion of the input/output signal waveforms and reducing the consumption power of the ICs.
(3) The capacitance between the anode and cathode is small for restricting distortion of the input/output signal waveforms of the ICs. For example, the capacitance should be less than about 10 pF.
(4) The withstand voltage for electrostatic charges is large.
With the conventional semiconductor devices shown in FIGS. 1 and 2, there arises a problem that the withstand voltage is not high enough when the capacitance is limited to a small value in order to restrict distortion of the input/output signal waveforms because the withstand voltage is in proportion to the capacitance.
In addition, when the input/output signal voltages of the ICs are 5 V or less, there arises another problem that the conventional semiconductor devices shown in FIGS. 1 and 2 cannot protect the ICs against the electrostatic charges. The problem is due to the fact that the breakdown of the main P-N junction is almost controlled by the Zener breakdown phenomenon to increase the leakage current when the breakdown voltage of the diode is 5 V or less. For example, in case of the breakdown voltage of 4 V, the leakage current becomes very large (for example, about several milliamperes) when the input/output signal voltages are 3 V or less.